Pulsed light synchronizer and microscope system

ABSTRACT

A pulsed light synchronizer synchronizes a first pulsed light having a first period and a second pulsed light and having a second period equal to the first period with each other. A third pulsed light is acquired by providing a first delay time between two pulsed lights acquired by dividing the first pulsed light, and by multiplexing the pulsed lights acquired from the first pulsed light. A fourth pulsed light is acquired by providing a second delay time between two pulsed lights acquired by dividing the second pulsed light, and by multiplexing the pulsed lights acquired from the second pulsed light. The pulsed light synchronizer detects, through a detector, a pulsed light acquired by multiplexing the third and fourth pulsed lights, and adjusts at least one of the first and second periods based on a timing difference between the third and fourth pulsed lights acquired from the detector.

TECHNICAL FIELD

The present invention relates to a pulsed light synchronizer and amicroscope system that synchronize timings of two pulsed lights (orpulsed light trains) emitted from two pulsed lasers.

BACKGROUND ART

In a nonlinear optical microscope, such as a stimulated Raman scatteringmicroscope, configured to exploit a nonlinear optical process, pulsedlights emitted by two pulsed lasers need to be focused on a sample withtimings of their pulses being synchronized (or a difference between thetimings being kept constant).

PLT 1 discloses a stimulated Raman scattering (SRS) microscope thatdetects, as a pulse timing difference, an output from a photodetectorthat detects two-photon absorption and adjusts a pulse period so thatthe detected value is equal to a set value. PLT 2 discloses a coherentanti-Stokes Raman scattering (CARS) microscope that adjusts a pulseperiod based on a difference between outputs from two photodetectorsused to detect a pulse timing difference.

CITATION LIST Patent Literature

[PLT1] Japanese Patent No. 5,501,360

[PLT2] Japanese Patent No. 4,862,164

SUMMARY OF INVENTION Technical Problem

In PLT 1, when the light intensities, wavelengths, and pulse durationsof the pulsed lights are changed, outputs from the photodetector and anoutput circuit thereof need to be set to different values to achieveequivalent pulsed light synchronization accordingly, but PLT 2 canachieve the pulsed light synchronization without such setting. However,PLT 2 requires such a configuration that the two photodetectors have thesame sensitivity with the same wavelength characteristic, and lightsinput to the photodetectors have the same light intensity and pulseduration. Otherwise, the pulsed light synchronization cannot beachieved, and pulses of the pulsed lights emitted from the two pulsedlasers have a timing difference when the wavelengths are changed.Moreover, since an output from a photodetector largely depends on thearrangement of a focusing objective lens and a light-receiving surfaceof the photodetector, the two photodetectors need to have identicalarrangements relative to the objective lens.

The present invention provides a pulsed light synchronizer and amicroscope system that can reliably operate.

Solution to Problem

A pulsed light synchronizer as an aspect of the present inventionsynchronizes a first pulsed light produced on a first period and asecond pulsed light produced on the first period with each other. Thepulsed light synchronizer includes a first delay multiplexer configuredto produce a third pulsed light by providing a first delay time betweentwo pulsed lights acquired by demultiplexing the first pulsed light, andby multiplexing the pulsed lights, a second delay multiplexer configuredto produce a fourth pulsed light by providing a second delay timebetween two pulsed lights acquired by demultiplexing the second pulsedlight, and by multiplexing the pulsed lights acquired from the secondpulsed light, a detector configured to detect a pulsed light acquired bymultiplexing the third pulsed light and the fourth pulsed light, aninformation acquirer configured to acquire information of a timingdifference between the third pulsed light and the fourth pulsed light byperforming synchronous detection on an output from the detector, and aperiod adjuster configured to adjust at least one of the first andsecond periods based on the information acquired by the informationacquirer.

Further features and aspects of the present invention will becomeapparent from the following description of exemplary embodiments withreference to the attached drawings.

Advantageous Effects of Invention

The present invention provides a pulsed light synchronizer and amicroscope system that can reliably operate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical path diagram of a pulsed light synchronizeraccording to an embodiment of the present invention.

FIG. 2A illustrates a time profile of the intensity of a third pulsedlight in the pulsed light synchronizer illustrated in FIG. 1.

FIG. 2B illustrates a time profile of the intensity of a fourth pulsedlight in the pulsed light synchronizer illustrated in FIG. 1.

FIG. 2C illustrates a time profile of the intensity of two-photonabsorption caused by the third and fourth pulsed lights in the pulsedlight synchronizer illustrated in FIG. 1.

FIG. 2D illustrates a time profile of the intensity of the fourth pulsedlight in the pulsed light synchronizer illustrated in FIG. 1.

FIG. 2E illustrates a time profile of the intensity of the two-photonabsorption caused by the third and fourth pulsed lights in the pulsedlight synchronizer illustrated in FIG. 1.

FIG. 2F illustrates a time profile of the intensity of the fourth pulsedlight in the pulsed light synchronizer illustrated in FIG. 1.

FIG. 2G illustrates a time profile of the intensity of the two-photonabsorption caused by the third and fourth pulsed lights in the pulsedlight synchronizer illustrated in FIG. 1.

FIG. 3 illustrates a relationship between an output voltage from asynchronous detection circuit and a timing difference between pulses.

FIG. 4 is a block diagram of a variation of the pulsed lightsynchronizer illustrated in FIG. 1.

FIG. 5A illustrates a time profile of the intensity of a first pulsedlight when the pulsed light synchronizer has the configurationillustrated in FIG. 4.

FIG. 5B illustrates a time profile of the intensity of the third pulsedlight when the pulsed light synchronizer has the configurationillustrated in FIG. 4.

FIG. 6 is a conceptual diagram of an SRS microscope using the pulsedlight synchronizer illustrated in FIG. 4 according to Embodiment 1.

FIG. 7A illustrates a time profile of the intensity of a first pulsedlight in the SRS microscope.

FIG. 7B illustrates a time profile of the intensity of a second pulsedlight in the SRS microscope.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is an optical path diagram of a pulsed light (train) synchronizer(hereinafter, simply referred to as a “synchronizer”) according to anembodiment of the present invention. The synchronizer synchronizes afirst pulsed light 21 emitted by a pulsed laser (first pulsed laser) 1and a second pulsed light 22 emitted by a pulsed laser (second pulsedlaser different from the first pulsed laser) 2. Specifically, thesynchronizer synchronizes timings of the emissions of the first pulsedlight 21 and the second pulsed light 22 or keeps constant a differencetherebetween. The first pulsed light 21 and the second pulsed light 22have wavelengths λ1 and λ2 different from each other, respectively.

The pulsed laser 2, whose cavity length is variable, can adjust thepulse period (second period) of the second pulsed light 22. The pulsedlaser 2 adjusts the second period depending on any timing shift betweenpulses so that the second period is synchronized with the pulse period(first period) of the first pulsed light 21, thereby synchronizing thetimings of the pulses. This produces the first pulsed light on the firstperiod, and produces the second pulsed light on the first period.

A half-mirror 3 transmits part of incident light, and reflects theremaining (part) of the incident light. The half-mirror 3 receives alight beam from the pulsed laser 1, transmitting part of the light beamin the right direction in FIG. 1 and reflecting the remaining thereof inthe downward direction in FIG. 1. The half-mirror 3 receives a lightbeam from the pulsed laser 2, transmitting part of the light beam in thedownward direction and reflecting the remaining in the right direction.The half-mirror 3 is disposed so that light beams from the pulsed lasers1 and 2 are each divided into the two directions, in each of whichdivided light beams from the pulsed lasers 1 and 2 are multiplexed onthe same axis. One pair of divided light beams are used in thesynchronizer, and the other pair of divided light beams are used in asystem, such as a nonlinear optical microscope, which requiressynchronized pulsed lights.

A half-mirror 4 is a beam splitter that receives light from thehalf-minor 3, transmitting the first pulsed light 21 and reflecting thesecond pulsed light 22. The half-mirror 4 is provided with a dielectricmultilayer film designed to transmit light having the wavelength λ1 andto reflect light having the wavelength λ2.

A light intensity adjuster 5 receives the first pulsed light 21transmitted through the half-mirror 4, and adjusts the light intensityof the first pulsed light 21. A method of the adjustment will bedescribed later. After transmitted through the light intensity adjuster5, the first pulsed light 21 enters a delay multiplexer 30.

A deflecting minor 6 bends (deflects), by 90 degrees, the optical pathof the second pulsed light 22 reflected by the half-mirror 4. A lightintensity adjuster 7 receives the second pulsed light 22 whose opticalpath is bent by the deflecting mirror 6, and adjusts the light intensityof the second pulsed light 22. A method of the adjustment will bedescribed later. After transmitted through the light intensity adjuster7, the second pulsed light 22 enters a delay multiplexer 40.

The delay multiplexer (first delay multiplexer) 30 includes polarizingbeam splitters (PBSs) 31 and 32 and minors 33 and 34. The PBSs eachsplit incident light depending on a polarization component thereof, forexample, transmitting the P-polarized component of the incident lightand separating the S-polarized component of the incident light into adirection orthogonal to the direction of the incident light.

Light entering the PBS 31 is divided into two pulsed lights havingpolarization states orthogonal to each other and traveling in twodifferent directions. One of the divided pulsed lights enters the PBS32, and the other pulsed light is reflected by the mirrors 33 and 34 andthen enters the PBS 32. The PBSs 31 and 32 and the minors 33 and 34 aredisposed at such angles that two pulsed lights emitted from the PBS 32are on the same axis. A pulsed light multiplexed through the PBS 32 isreferred to as a third pulsed light 23.

FIG. 2A illustrates a time profile of the intensity of the third pulsedlight 23. In FIG. 2A, the horizontal axis represents time (t), and thevertical axis represents the light intensity, and this arrangement isalso applied in FIGS. 2B to 2G.

FIG. 2A illustrates pulses due to the two pulsed lights divided throughthe PBS 31, respectively with a solid line and a dotted line. Since thetwo divided pulsed lights travel on optical paths having lengthsdifferent from each other, the pulses illustrated with the solid lineand the dotted line have a time shift therebetween by a first delay timeT1. The light intensity adjuster 5 adjusts the polarization of the firstpulsed light 21 so that the pulsed lights illustrated with the solidline and the dotted line have equal intensities. The light intensityadjuster 5 is, for example, a half-wave plate, and adjusts an intensityratio of the pulsed lights divided through the PBS 31 by rotating thedirection of the polarization entering the delay multiplexer 30.

Typically, once performed, the adjustment of the light intensityadjuster 5 does not need to be performed constantly, unlike feedbackcontrol. For example, to acquire the information illustrated in FIG. 2A,a dedicated photodetector is placed at a position at which a deflectingmirror 8 is to be installed, and the light intensity of at least one ofthe two divided pulsed lights is adjusted so that the two divided pulsedlights have equal light intensities after multiplexed. For example, thehalf-wave plate may be rotated around an optical axis illustrated with adashed and single-dotted line in FIG. 1 so that two adjacent pulses haveequal light intensities. The half-wave plate may be rotated manually bya maintainer so that two adjacent pulses have equal light intensities,or a rotator of the half-wave plate may be controlled by anunillustrated controller so that two adjacent pulses have equal lightintensities. Alternatively, depending on the sensitivity of a detector(photodetector) 11 described later, power supply to the pulsed laser 2may be cut or the second pulsed light 22 may be shielded, and thehalf-wave plate may be rotated around the optical axis so that an outputfrom a synchronous detection circuit 12 described later is zero.

The PBSs 31 and 32 may be each replaced with a half-mirror that equallydivides the light intensity of incident light. In this case, in place ofthe light intensity adjuster 5, an element (variable ND filter, forexample) that adjusts the light intensity is introduced into one ofoptical paths bifurcating at the half-mirror.

Similarly, the delay multiplexer (second delay multiplexer) 40 includesPBSs 41 and 42 and mirrors 43 and 44. Light entering the PBS 41 isdivided into two pulsed lights having polarization states orthogonal toeach other and traveling in two different directions. One of the dividedpulsed lights enters the PBS 42, and the other pulsed light is reflectedby the mirrors 43 and 44 and then enters the PBS 42. The PBSs 41 and 42and the mirrors 43 and 44 are disposed at such angles that so that twopulsed lights emitted from the PBS 42 are on the same axis. A pulsedlight multiplexed through the PBS 42 is referred to as a fourth pulsedlight 24.

FIG. 2B illustrates a time profile of the intensity of the fourth pulsedlight 24. FIG. 2B illustrates pulses due to the two pulsed lightsdivided through the PBS 41 respectively with a solid line and a dottedline. Since the two divided pulsed lights travel on optical paths havinglengths different to each other, the pulses illustrated with the solidline and the dotted line have a time shift therebetween by a seconddelay time T2. The second delay time T2 has such a relationship with thefirst delay time T1 that is represented by Expression (1). The lightintensity adjuster 7 adjusts the polarization of the second pulsed light22 so that the pulsed lights illustrated with the solid line and thedotted line have equal intensities. The light intensity adjuster 7 is,for example, a half-wave plate, and adjusts an intensity ratio of thepulsed lights divided through the PBS 41 by rotating the direction ofthe polarization entering the delay multiplexer 40.

Typically, once performed, the adjustment of the light intensityadjuster 7 does not need to be performed constantly, unlike feedbackcontrol. For example, to acquire the information illustrated in FIG. 2B,a dedicated photodetector is placed at a position at which a half-mirror9 is to be installed. Then, the light intensity of at least one of thetwo divided pulsed lights is adjusted so that the two divided pulsedlights have equal light intensities after multiplexed. For example, thehalf-wave plate is rotated around an optical axis illustrated with adotted line in FIG. 1 so that two adjacent pulses have equal lightintensities. The half-wave plate may be rotated manually by a maintainerso that two adjacent pulses have equal light intensities, or a rotatorof the half-wave plate may be controlled by an unillustrated controllerso that two adjacent pulses have equal light intensities. Alternatively,depending on the sensitivity of the photodetector 11 described later,power supply to the pulsed laser 1 may be cut or the first pulsed light21 may be shielded, and the half-wave plate may be rotated around theoptical axis so that an output from the synchronous detection circuit 12described later is zero.

The PBSs 41 and 42 may be each replaced with a half-mirror that equallydivides the light intensity of incident light. In this case, in place ofthe light intensity adjuster 7, an element (variable ND filter, forexample) that adjusts the light intensity is introduced on one ofoptical paths bifurcating at the half-minor.

The third pulsed light 23 emitted from the delay multiplexer 30 isreflected by the deflecting minor 8 so that the optical path of thethird pulsed light 23 is bent by 90 degrees, and then enters thehalf-minor 9. The third pulsed light 23 emitted from the delaymultiplexer 30 and the fourth pulsed light 24 emitted from the delaymultiplexer 40 are multiplexed on the same axis through the half-mirror9. The half-mirror 9 has the same design as that of the half-mirror 4,and transmits light having the wavelength λ1 and reflects light havingthe wavelength λ2.

The third pulsed light 23 and the fourth pulsed light 24 that aremultiplexed are focused on a light-receiving surface of thephotodetector 11 through an objective lens 10. The objective lens 10 mayhave a numerical aperture of 0.5 or higher to obtain a large two-photonabsorption signal detected at the photodetector 11.

The photodetector 11 includes, for example, a photodiode that receivesthe third pulsed light and the fourth pulsed light. The photodetector 11converts a current produced through two-photon absorption at thelight-receiving surface of the photodiode into a voltage. To obtain atwo-photon absorption signal, the photodiode included in thephotodetector 11 has sensitivity at a wavelength λ1·λ2/(λ1+λ2)corresponding to a sum of photon energy (E3∝1/λ1) of the third pulsedlight 23 and photon energy (E4∝1/λ2) of the fourth pulsed light 24, thatis, E3+E4. When the third pulsed light 23 has a wavelength of 800 nm andthe fourth pulsed light 24 has a wavelength of 1030 nm, a sensitivityfor light detection needs to be at 450 nm.

FIG. 2C illustrates a time profile of the two-photon absorption signal(current produced by the photodiode) corresponding to E3+E4 when thethird pulsed light 23 is in the state illustrated in FIG. 2A and thefourth pulsed light 24 is in the state illustrated in FIG. 2B. Thetwo-photon absorption signal is proportional to the product of the pulseintensity of the third pulsed light 23 and the pulse intensity of thefourth pulsed light 24.

FIGS. 2A and 2B each illustrate a state in which the pulsed lights aresynchronized, and a timing difference between pulse peaks of the thirdpulsed light 23 and the fourth pulsed light 24 is equal between adjacentpulses (Δt1 and Δt2 in FIG. 2B are equal). This pulse synchronizationstate is achieved by controlling a pulse period adjuster 14 (or thecavity length of the pulsed laser 2) such that intensities of adjacentpulses of the two-photon absorption signal are equal as illustrated inFIG. 2C.

FIG. 2D illustrates a time profile of the intensity of the fourth pulsedlight 24 when the cavity length of the pulsed laser 1 or 2 has changeddue to disturbance, and the fourth pulsed light 24 has reached thephotodetector 11 later than the fourth pulsed light 24 in the pulsesynchronization state.

FIG. 2E illustrates a time profile of the two-photon absorption signalcorresponding to E3+E4 when the third pulsed light 23 and the fourthpulsed light 24 are respectively in the states illustrated in FIG. 2Aand FIG. 2D. In this case, adjacent pulses of the two-photon absorptionsignal have different intensities.

FIG. 2F illustrates a time profile of the intensity of the fourth pulsedlight 24 when the cavity length of the pulsed laser 1 or 2 has changeddue to disturbance, and the fourth pulsed light 24 has reached at thephotodetector 11 faster than the fourth pulsed light 24 in the pulsesynchronization state.

FIG. 2G illustrates a time profile of the two-photon absorption signalcorresponding to E3+E4 when the third pulsed light 23 and the fourthpulsed light 24 are respectively in the states illustrated in FIG. 2Aand FIG. 2F. Similarly to FIG. 2E, adjacent pulses of the two-photonabsorption signal have different intensities, but the intensities areinverted between FIG. 2E and FIG. 2F. Delay and advance of a pulsedlight can be known by detecting this inversion of the intensities.

The photodetector 11 may include a nonlinear crystal (barium boratecrystal, for example) that receives the third pulsed light 23 and thefourth pulsed light 24, and a photomultiplier that detects a sumfrequency light produced through the nonlinear crystal. Similarly to thetwo-photon absorption signal corresponding to E3+E4, the sum frequencylight has an intensity difference indicating the delay and advance of apulsed light.

The intensity difference between adjacent pulses of the two-photonabsorption signal is evaluated through synchronous detection describedlater, and thus the first delay time T1 and the second delay time T2 areset to be approximately half of the first (or second) pulse period. Whena difference between T1 and T2 is zero, that is, when T1 and T2 areequal to each other, the delay and advance of a pulsed light cannot beevaluated because the intensity difference between adjacent pulses of asignal of interest is zero. When an absolute value of the differencebetween T1 and T2 is larger than a sum (T1+T2) of first and second pulsedurations, the two-photon absorption signal is small and the intensitydifference between adjacent pulses cannot be evaluated. Thus, theabsolute value of the difference between T1 and T2 is required to benon-zero and smaller than or equal to the sum (τ1+τ2) of the first andsecond pulse durations. This requirement is equivalent to satisfying acondition below.

0<|T1−T2| (τ1+τ2)   (1)

The first pulse duration is a full width at half maximum (half width) ofeach light pulse of the first pulsed light. The second pulse duration isa full width at half maximum of each light pulse of the second pulsedlight. When the pulse synchronizer according to the embodiment of thepresent invention is reliably operated, each of the pulse durations maybe approximately one to three times as large as the full width at halfmaximum.

T1 can be adjusted by changing the positions of the mirrors 33 and 34,and T2 can be adjusted by changing the positions of the minors 43 and44. The intensity difference of the two-photon absorption signal has aperiod equal to the pulse period (second period) of the second pulsedlight from which the fourth pulsed light is produced.

The synchronous detection circuit (information acquirer) 12 is anelectric circuit such as a lock-in amplifier. The synchronous detectioncircuit 12 acquires, though synchronous detection, information of theamplitude on the second period (information of a timing differencebetween the third pulsed light and the fourth pulsed light), which isincluded in the output voltage of the photodetector 11, and outputs theinformation as a voltage. Since the second period and the first periodare substantially equal to each other (completely synchronized throughpulse synchronization), the synchronous detection circuit 12 may beconfigured to acquire information of only the amplitude of the firstperiod (instead of the second period) from the output voltage of thephotodetector 11.

The product of an input signal (sin α) and a reference signal (sin β)having the same frequency and phase as those of the input signal yieldssin α·sin β={cos(α−β)−cos(α+β)}/2 using the trigometric identity, whichis then written as {cos(0)−cos(2α)}/2 because α=β is established. Thisincludes a direct current component proportional to the amplitude of theinput signal and an alternate current component having a frequency twiceas that of the input signal, and thus the direct current component forthe input signal is obtained by removing the alternate current componentthrough a lowpass filter.

FIG. 3 is a graph illustrating a relationship between the output voltage(vertical axis) of the synchronous detection circuit 12, and a timingshift (horizontal axis) of the fourth pulsed light 24 from the fourthpulsed light in the pulse synchronization state (that is, a timingdifference between the third pulsed light 23 and the fourth pulsed light24).

The direction (delay or advance of the fourth pulsed light 24) of thetiming difference between pulses corresponds to the sign of the outputvoltage of the synchronous detection circuit 12. The output voltage ofthe photodetector 11 includes, in addition to a component due to thetwo-photon absorption corresponding to E3+E4, components due totwo-photon absorption in the third and fourth pulsed lights themselvessuch as E3+E3 and E4+E4, and components due to one-photon absorptionsuch as E3 and E4. However, the components other than the componentcorresponding to E3+E4 are each produced on a period half of the secondperiod, and thus do not affect the output from the synchronous detectioncircuit 12. The light intensity ratio of adjacent pulses illustrated inFIG. 2C corresponds to an intersection point (where the output voltageis zero) in FIG. 3, and the light intensity ratio of adjacent pulsesillustrated in FIGS. 2E and 2G corresponds to regions in which theoutput voltage is positive and negative in FIG. 3.

A feedback circuit 13 outputs a voltage to be applied to the pulseperiod adjuster 14 installed in the pulsed laser 2 to correct a timingdifference between pulses corresponding to the output voltage of thesynchronous detection circuit 12.

When the output voltage of the synchronous detection circuit 12 ispositive, for example, the fourth pulsed light 24 is assumed to be laterthan the third pulsed light 23. In this case, the feedback circuit 13outputs a voltage for reducing the cavity length of the pulsed laser 2to advance the fourth pulsed light 24 so that the fourth pulsed light 24can be closer to the synchronization state. When the output voltage ofthe synchronous detection circuit 12 is negative, the feedback circuit13 outputs a voltage for increasing the cavity length. These processesare such a feedback control that the output voltage of the synchronousdetection circuit 12 is made zero, and can achieve the pulsesynchronization state illustrated in FIGS. 2A and 2B. The feedbackcircuit 13 may be configured such that the output voltage of thesynchronous detection circuit 12 is non-zero, so as to give a desiredtiming difference between the third pulsed light 23 and the fourthpulsed light 24.

The pulse period adjuster 14 includes a stage to which a phase modulatoror mirror is attached, and the cavity length of the pulsed laser 2 isadjusted by applying a voltage to the phase modulator and driving thestage. The pulse period adjuster 14 may be installed not in the pulsedlaser 2 but in the pulsed laser 1 so as to adjust the cavity length ofthe pulsed laser 1.

The third pulsed light 23 and the fourth pulsed light 24 aresynchronized to be respectively in the states illustrated in FIGS. 2Aand 2B at the light-receiving surface of the photodetector 11. Beforesynchronized, timings of pulse peaks of the third pulsed light 23 andthe fourth pulsed light 24 are slightly shifted from each other at thelight-receiving surface of the photodetector 11. In light beams dividedthrough the half-mirror 3 and used in, for example, a nonlinear opticalmicroscope, peaks of the first and second pulsed lights can besynchronized by adjusting the optical path length from the half-mirror 3to the photodetector 11 to be different between the third and fourthpulsed lights so as to correct the shift.

The sign of the output voltage of the synchronous detection circuit 12tells whether the third pulsed light 23 is delayed or advanced relativeto the fourth pulsed light 24. When light intensities and pulsedurations change, only the absolute value of the output voltage of thesynchronous detection circuit 12 changes and the sign thereof does notchange, and thus pulsed light (train) synchronization is achieved.Similarly, when wavelengths change in the range of the sensitivity ofthe photodetector 11, the pulsed light synchronization is achieved.Since only one photodetector is used, two photodetectors does not needto have identical arrangements relative to the objective lens unlike PLT2.

In this embodiment, pulsed lights are each divided through thehalf-minor 4 into pulsed lights having two wavelengths after multiplexedthrough the half-mirror 3, but the first and second pulsed lights may beeach divided through a half-mirror before multiplexed through thehalf-mirror 3. In this embodiment, the delay multiplexer 30 and thedelay multiplexer 40 assumes the use of light propagating in space, butmay use light propagating in an optical fibre.

In this embodiment, the pulse periods of the two pulsed lights have aratio of 1:1, but may have a ratio of 1:2. In other words, the secondpulsed light may be produced on a period twice as that of the firstperiod. FIG. 4 is a block diagram of the synchronizer when the pulseperiods of the two pulsed lights have a ratio of 1:2. In a configurationillustrated in FIG. 4, timings of pulses are synchronized for the firstpulsed light 21 and a second pulsed light 25 produced by a pulsed laser15 and having a third pulse duration T3 and a third pulse period. Theconfiguration is largely different from the configuration illustrated inFIG. 1 in that only one, instead of two, delay multiplexer is employed,and only the second pulsed light 25 is transmitted through the delaymultiplexer.

The first pulsed light 21 transmitted through the half-mirror 4 isreflected by the deflecting minor 8 without being transmitted through adelay multiplexer, and then enters the half-mirror 9. FIG. 5Aillustrates a time profile of the intensity of the first pulsed light 21at the light-receiving surface of the photodetector 11. The first pulsedlight 21 has a pulse period P1. In FIG. 5A, the horizontal axisrepresents time (t), and the vertical axis represents the lightintensity, and this arrangement is also applied in FIG. 5B.

The second pulsed light 25 reflected by the half-minor 4 is reflected bythe deflecting minor 6, transmitted through the light intensity adjuster7, and enters a delay multiplexer 60. The delay multiplexer 60 includesPBSs 61 and 62 and minors 63 and 64. Light entering the PBS 61 isdivided into two pulsed lights having polarization states orthogonal toeach other and traveling in two different directions. One of the dividedpulsed lights enters the PBS 62, and the other pulsed light is reflectedby the minors 63 and 64, and then enters the PBS 62. The PBSs 61 and 62and the mirrors 63 and 64 are disposed at such angles that two pulsedlights emitted from the PBS 62 are on the same axis.

The pulsed lights multiplexed through the PBS 62 are referred to as athird pulsed light 26. FIG. 5B illustrates a time profile of theintensity of the third pulsed light 26 at the light-receiving surface ofthe photodetector 11, and pulses due to the two pulsed lights dividedthrough the PBS 61 respectively with a solid line and a dotted line.Since the two divided pulsed lights travel on optical paths havinglengths different to each other, the pulses illustrated with the solidline and the dotted line have a time shift therebetween by a third delaytime T3.

The absolute value of a difference between the pulse period P1 of thefirst pulsed light 21 and the third delay time T3 is set to be non-zeroand smaller than or equal to the sum of the first and third pulsedurations (τ1+τ3). This is equivalent to satisfying a condition below.

0<|P1−T3|(τ1+τ3)   (2)

The first pulse duration τ1 is equal to the full width at half maximum(half width) of each pulse of the first pulsed light 21. The third pulseduration τ3 is equal to the full width at half maximum (half width) ofeach pulse of the second pulsed light 25. When the pulse synchronizeraccording to the embodiment of the present invention is reliablyoperated, each of the pulse durations may be approximately one to threetimes as large as the full width at half maximum.

Typically, once performed, the adjustment of the light intensityadjuster 7 does not need to be performed constantly, unlike feedbackcontrol. For example, to acquire the information illustrated in FIG. 5B,a dedicated photodetector is placed at a position at which thehalf-mirror 9 is to be installed. Then, the light intensity of at leastone of the two divided pulsed lights is adjusted so that the two dividedpulsed lights have equal light intensities after multiplexed. Forexample, the half-wave plate is rotated around an optical axisillustrated with a dotted line in FIG. 4 so that two adjacent pulseshave equal light intensities. The half-wave plate may be rotatedmanually by a maintainer so that two adjacent pulses have equal lightintensities, or a rotator of the half-wave plate may be controlled by anunillustrated controller so that two adjacent pulses have equal lightintensities. Alternatively, depending on the sensitivity of thephotodetector 11, power supply to the pulsed laser 1 may be cut, thefirst pulsed light 21 may be shielded, and the half-wave plate may berotated around the optical axis so that an output from the synchronousdetection circuit 12 described later is zero.

The first pulsed light 21 and the third pulsed light 26 multiplexedthrough the half-mirror 9 are focused on the light-receiving surface ofthe photodetector 11 through the objective lens 10. The synchronousdetection circuit 12 acquires the amplitude (information of a timingdifference between the first pulsed light and the third pulsed light) ofthe third pulse period component included in an output voltage of thephotodetector 11, and outputs the amplitude as voltage. The outputvoltage of the synchronous detection circuit 12 reflects a pulse timingdifference between the first pulsed light 21 and the third pulsed light26. To correct the pulse timing difference, the feedback circuit 13outputs the voltage to a pulse period adjuster 16, so that the firstpulsed light 21 and the third pulsed light 26 are controlled to be inthe states illustrated in FIGS. 5A and 5B, respectively.

The present invention is also applicable to a pulsed light synchronizerfor two pulsed lights having pulse periods whose ratio is m:n. m and nare reduced integers. When m and n are both odd, similarly to the casein which the pulse periods are 1:1, a delay multiplexer is provided foreach pulsed light. Similarly to the case in which the pulse periods are1:1, the pulses can be synchronized by setting a delay time so that atime difference between the pulsed lights produced with delay is notlarger than the sum of the widths of the pulses.

When m and n are a combination of an odd number and an even number,similarly to the case in which the pulse periods are 1:2, a delaymultiplexer is provided only for the pulsed light for the odd number mor n. Similarly to the case in which the pulse periods are 1:2, thepulses can be synchronized by setting a delay time so that a timedifference between the pulsed light produced with delay and the otherpulsed light for which no delay multiplexer is provided is not largerthan the sum of the widths of the pulses.

Embodiment 1

FIG. 6 is a block diagram of a microscope system according to Embodiment1 of the present invention. The microscope system includes an SRSmicroscope 100 and a pulsed light synchronizer 200. The SRS microscope100 multiplexes two pulsed lights emitted from the two pulsed lasers 1and 15 and having different wavelengths and detects stimulated Ramanscattering (SRS) light produced by focusing and simultaneously emittingthe pulses on a sample 105. Thus, the SRS microscope 100 is a nonlinearoptical microscope that irradiates the sample with the two pulsed lightsemitted from the two pulsed lasers and having different wavelengths, andobserves the sample through a nonlinear optical process. The pulsedlight synchronizer 200 synchronizes pulsed lights emitted from the twopulsed lasers 1 and 15.

The SRS is a nonlinear optical phenomenon that occurs proportionally tothe product of the intensities of lights having the differentwavelengths. To efficiently produce the SRS, laser light beams havingtwo wavelengths are focused at an identical position, and pulsed lightshaving the two wavelengths are synchronized so that the pulsed lightsare simultaneously focused. When the SRS is produced, among the pulsedlights having the two wavelengths, the intensity of the pulsed lighthaving a shorter wavelength is reduced, and the intensity of the pulsedlight having a longer wavelength is increased. To efficiently producethe SRS, a pulsed laser having a pulse duration of 1 to 10 picosecondsis desirably used.

The pulsed lasers 1 and 15 produce pulsed lights whose pulse periods are1:2. FIG. 7A illustrates the first pulsed light 21 produced by thepulsed laser 1, and FIG. 7B illustrates the second pulsed light 25produced by the pulsed laser 15. In FIGS. 7A and 7B, the horizontal axisrepresents time (t), and the vertical axis represents the lightintensity. The wavelength (λ1) of the first pulsed light 21 is smallerthan the wavelength (λ2) of the second pulsed light 25.

A solid-state laser (titanium-sapphire laser) having a centralwavelength of 800 nm and a pulse period of 12.5 nanoseconds is used asthe pulsed laser 1. For example, Mai Tai, which is manufactured bySpectra-Physics, is used. A ytterbium-doped fibre laser having a centralwavelength of 1030 nm and a pulse period of 25 nanoseconds is used asthe pulsed laser 15.

When pulses of the first pulsed light 21 and the second pulsed light 25are focused at an identical position on the sample at synchronizedtimings as illustrated in FIGS. 7A and 7B, the light intensity of apulsed light transmitted through the sample by the SRS is changed. Thelight intensities of pulses 1, 3, and 5 in FIG. 7A are reduced, and thelight intensities of pulses 2 and 4 are not changed. Since thisdifference between the light intensities of adjacent pulses is minute,the difference is detected through synchronous detection.

The detected difference between the light intensities corresponds to anSRS signal, and information of molecules at a position where the lightbeams are focused is reflected on the SRS signal. For example, when theresonance frequency of the molecular vibration at the position is equalto a difference between the light frequencies of the two lasers(c/λ1-c/λ2), the SRS signal becomes large. c represents the speed oflight. The Raman spectrum can be acquired by acquiring the SRS signalwhile the difference between the light frequencies of the two lasers(c/λ1-c/λ2) is changed. The Raman spectrum allows estimation of whatkinds of molecules are included in the sample. The SRS microscope canacquire a spectrum at the same level as that of a microscope exploitingspontaneous Raman scattering. Since the SRS has a scattering efficiencyextremely larger than that of the spontaneous Raman scattering, the SRSmicroscope can acquire the Raman spectrum in a shorter time than themicroscope exploiting the spontaneous Raman scattering. The wavelengthof at least one of the two lasers is changed to obtain the Ramanspectrum. Since the change of the wavelength changes the sensitivity ofthe photodetector, the microscope system of Embodiment 1, which reliablysynchronizes pulsed lights irrespective of the change of the wavelength,is applicable as the SRS microscope.

The half-minor 3 multiplexes light beams emitted from the pulsed lasers1 and 15 onto the same axis, and divides the light beams in twodirections. One of the divided light beams enters the pulsed lightsynchronizer 200, and the other enters the SRS microscope 100. Thepulsed light synchronizer 200 synchronizes pulsed lights of two pulsedlasers entering the SRS microscope 100.

The SRS microscope 100 has a configuration of a laser scanningmicroscope. Light beams of the two pulsed lasers enter a beam scanner101 on the same axis, and deflected and emitted by the beam scanner 101.The beam scanner 101 includes a galvano scanner and a resonant scanner,and changes the direction of the optical axis of the light beams intotwo directions orthogonal to each other. For simplification, FIG. 6illustrates the two mirrors in the beam scanner 101 representatively asone minor. The use of the resonant scanner (having a scanning frequencyof 8 kHz) and the galvano scanner (having a scanning frequency of 15 Hz)allows an image of 500 lines to be acquired at 30 frames per second.

The light beams deflected by the beam scanner 101 enter an objectivelens 104 though lenses 102 and 103. The lenses 102 and 103 disposed sothat the beam scanner 101 and an entrance pupil of the objective lens104 are conjugate with each other allow the light beams deflectedthrough the beam scanner 101 to be focused on the sample 105 with theirlight being not shielded. The magnification of an optical system throughthe lenses 102 and 103 is selected so that the size of the entrancepupil of the objective lens 104 is equal to the size of the incidentlight beam. This minimizes the size of a light spot focused through theobjective lens 104 and improves a spatial resolution of detecting theSRS signal. Since an increased intensity of the light spot enhances theSRS signal, leading to an improved signal-to-noise ratio (S/N ratio) ofthe detection of the SRS signal. The objective lens 104 desirably has alarger numerical aperture (NA) in terms of the spatial resolution ofdetecting the SRS signal and the S/N ratio.

The sample 105 is sandwiched between cover glasses (not illustrated)each having a thickness of several 10 s to 200 micrometers.Two-dimensional scanning of the light spot focused on the sample 105 isperformed through the deflection of the light beams through the beamscanner 101, and produces the SRS signal as a two-dimensional image.Since the SRS signal is produced only at the focused light spot, athree-dimensional image can be obtained by moving the sample 105 on astage (not illustrated) in the direction of the optical axis.

To thoroughly receive light transmitted through the sample 105 andprovided with an intensity modulation through the SRS, the objectivelens 106 has a numerical aperture (NA) equivalent to or higher than thatof the objective lens 104. Light beams emitted from the objective lens106 are transmitted through a filter 107 and a lens 108 and then madeincident on the light-receiving surface of a photodiode 109. The filter107 includes a dielectric multilayer film that shields light having thewavelength λ2 and transmits light having the wavelength λ1. Thephotodiode 109 is irradiated with the pulsed light that is emitted fromthe pulsed laser 1 and repeats the intensity modulation through the SRSat each pulse. A silicon photodiode having a sensitivity to a pulsedlight of 800 nanometers and having a cutoff frequency of 40 MHz orhigher is used as the photodiode 109.

The pulsed light 21 from the pulsed laser 1 has a repetition frequencyof 80 MHz (on a pulse period of 12.5 nanoseconds), whereas the intensitymodulation through the SRS has a repetition frequency of 40 MHz (on aperiod of 25 nanoseconds). A current-voltage converter 110 is anelectric circuit for outputting, as a voltage signal, a current signalproduced by the photodiode 109.

A synchronous detection circuit 111 extracts the amplitude of acomponent at 40 MHz from the voltage signal output from thecurrent-voltage converter 110 and outputs the amplitude as a voltage. Amixer circuit or a lock-in amplifier is used as the synchronousdetection circuit 111. The output voltage from the synchronous detectioncircuit 111 indicates the degree of the SRS at a focus point on thesample 105.

A computer 112 displays a two-dimensional image of output signals (SRSsignals) of the synchronous detection circuit 111 that are acquired byusing a signal for controlling the beam scanner 101. The computer 112can also display a three-dimensional image of SRS signals acquired bymoving the sample 105 on the stage (not illustrated) in the direction ofthe optical axis. The computer 112 can also display a Raman spectrumusing SRS signals acquired by changing the wavelength of at least one oftwo pulsed lasers.

The present invention is also applicable to a pulsed lightsynchronization method of synchronizing the first pulsed light havingthe first pulse duration and the first period, and the second pulsedlight having the second pulse duration and the second period equal to ortwice as the first period. The pulsed light synchronization method maybe realized as a program that causes a computer to execute steps.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-165643, filed on Aug. 18, 2014 which is hereby incorporated byreference herein in its entirety.

INDUSTRIAL APPLICABILITY

The present invention is applicable in coinciding timings of pulses oftwo pulsed lights emitted from two pulsed lasers.

REFERENCE SIGNS LIST

1 . . . pulsed laser (first pulsed laser), 2 and 15 . . . pulsed laser(second pulsed laser), 11 . . . photodetector, 12 . . . synchronousdetection circuit (information acquirer), 30 . . . first delaymultiplexer, 40 . . . second delay multiplexer, 200 . . . pulsed lightsynchronizer

1. A pulsed light synchronizer configured to synchronize a first pulsedlight produced on a first period and a second pulsed light produced onthe first period with each other, the pulsed light synchronizercomprising: a first delay multiplexer configured to produce a thirdpulsed light by providing a first delay time between two pulsed lightsacquired by dividing the first pulsed light, and by multiplexing thepulsed lights acquired from the first pulsed light; a second delaymultiplexer configured to produce a fourth pulsed light by providing asecond delay time between two pulsed lights acquired by dividing thesecond pulsed light, and by multiplexing the pulsed lights acquired fromthe second pulsed light; a detector configured to detect a pulsed lightacquired by multiplexing the third pulsed light and the fourth pulsedlight; an information acquirer configured to acquire information of atiming difference between the third pulsed light and the fourth pulsedlight based on an output from the detector; and a period adjusterconfigured to adjust at least one of the first and second periods basedon the information acquired by the information acquirer.
 2. The pulsedlight synchronizer according to claim 1, wherein a condition below issatisfied:0<|T1−T2|<(τ1+τ2) where τ1 represents a pulse duration of the firstpulsed light, τ2 represents a pulse duration of the second pulsed light,T1 represents the first delay time, and T2 represents the second delaytime.
 3. The pulsed light synchronizer according to claim 1, furthercomprising a light intensity adjuster configured to adjust at least oneof light intensities of the two pulsed lights acquired through divisionso that the light intensities of the two pulsed lights thus adjusted canbecome equal to each other after being multiplexed in at least one ofthe first delay multiplexer and the second delay multiplexer.
 4. Thepulsed light synchronizer according to claim 1, wherein the detectorincludes a photodiode configured to receive the third pulsed light andthe fourth pulsed light and to convert a current produced throughtwo-photon absorption into a voltage at a light-receiving surface of thephotodiode.
 5. The pulsed light synchronizer according to claim 1,wherein the detector includes a nonlinear crystal configured to receivethe third pulsed light and the fourth pulsed light, and aphotomultiplier configured to detect a sum frequency light producedthrough the nonlinear crystal.
 6. The pulsed light synchronizeraccording to claim 1, wherein the information acquirer is configured toacquire information of the timing difference between the third pulsedlight and the fourth pulsed light by performing synchronous detection onan output from the detector.
 7. A pulsed light synchronizer configuredto synchronize a first pulsed light produced on a first period and asecond pulsed light produced on a second period that is twice as long asthe first period with each other, the pulsed light synchronizercomprising: a delay multiplexer configured to produce a third pulsedlight by providing a delay time between two pulsed lights acquired bydividing the second pulsed light, and by multiplexing the pulsed lightsacquired from the first pulsed light; a detector configured to detect apulsed light acquired by multiplexing the first pulsed light and thethird pulsed light; an information acquirer configured to acquireinformation of a timing difference between the first pulsed light andthe third pulsed light based on an output from the detector; and aperiod adjuster configured to adjust at least one of the first andsecond periods based on the information acquired by the informationacquirer.
 8. The pulsed light synchronizer according to claim 7, whereina condition below is satisfied:0<|P1−T3|<(τ1+τ3) where τ1 represents a pulse duration of the firstpulsed light, τ3 represents a pulse duration of the second pulsed light,P1 represents the first period, and T3 represents the delay time.
 9. Thepulsed light synchronizer according to claim 7, further comprising alight intensity adjuster configured to adjust at least one of lightintensities of the two pulsed lights acquired through division so thatthe light intensities of the two pulsed lights can become equal to eachother after being multiplexed in the delay multiplexer.
 10. The pulsedlight synchronizer according to claim 7, wherein the detector includes aphotodiode configured to receive the first pulsed light and the thirdpulsed light and to convert a current produced through two-photonabsorption into a voltage at a light-receiving surface of thephotodiode.
 11. The pulsed light synchronizer according to claim 7,wherein the detector includes a nonlinear crystal configured to receivethe first pulsed light and the third pulsed light, and a photomultiplierconfigured to detect a sum frequency light produced through thenonlinear crystal.
 12. The pulsed light synchronizer according to claim7, wherein the information acquirer is configured to acquire informationof the timing difference between the first pulsed light and the thirdpulsed light by performing synchronous detection on an output from thedetector.
 13. A microscope system comprising: the pulsed lightsynchronizer according to claim 1; and a nonlinear optical microscopeconfigured to irradiate a sample with the first and second pulsed lightsand to observe the sample through a nonlinear optical process.